A cardiac assist system for delivering a multiplicity of pacing pulses to a heart within a period of less than about 100 milliseconds, and for varying the properties of such pulses as the response of a heart to them varies.
Cardiac assist devices, commonly referred to as pacemakers, comprise a wide range of functions but share a need to provide stimulus to the heart in order to initiate a heartbeat, to eliminate arrhythmia, or to defibrillate the heart. Traditional cardiac assist devices rely on electronic means to stimulate the heart and to monitor the presence of a normal or abnormal heartbeat. These devices typically use a battery as the source of electrical power, and much development effort has been expended in increased battery capacity and in improvements in the energy efficiency of the pacemaker systems. Further improvements in energy efficiency would benefit both the device manufacturer and the implant patient.
To the best of applicant""s knowledge, none of the prior art cardiac assist devices provide means for achieving this efficiency goal. It is an object of this invention to provide a device for improving the energy efficiency of a cardiac assist device while simultaneously monitoring the response of a heart to such device and adjusting the output of such device in response to such monitoring.
Many prior art patents disclose means for sensing the response of a heart to the input from a pacemaker. Thus, e.g., by way of illustration and not limitation, U.S. Pat. No. 5,957,857 discloses an improved automatic sensing system for an implantable pacemaker in which the sensing threshold is automatically set to optimally sense the P-wave or R-wave while rejecting noise. The invention of this patent is limited to rather traditional noise filtering and rejection techniques, and addresses the need to sense heart function at a relatively low speed; on the order of the second beat interval of the heart. The entire disclosure of this United States patent is hereby incorporated by reference into this specification.
U.S. Pat. No. 6,144,881 similarly discloses means to improve the detection of evoked potential caused by the heartbeat, by selectively rejecting polarization potential that is a byproduct of the stimulating signal sent by the pacemaker. However, the invention of this patent also restricts the sensing of heartbeat, and the attendant problem of noise rejection to the period after the pacing signal has been sent to the heart. While this patent suggests more advanced nonlinear filtering techniques and adaptive response to changes in heart response to long term pacing, it still does not provide any means for detecting the response of the heart prior to the end of the pacing signal. The entire disclosure of this United States patent is hereby incorporated by reference into this specification.
U.S. Pat. No. 5,871,512 discloses the use of digital signal processing to detect specific signal artifacts sensed by a pacemaker and specifically relating to a movement of electrical potential in a negative direction. However, as with the foregoing inventions, this measurement and analysis is done during a period in time after the entire pacing signal to the heart has been terminated. The entire disclosure of this United States patent is hereby incorporated by reference into this specification.
U.S. Pat. No. 5,330,512 deals in similar fashion with the problem of measuring evoked potential caused by the heartbeat in the presence of much higher polarization potential in the cardiac tissues immediately disposed around the pacing electrodes. It farther suggests the use of an additional electrode in a manner that permits the pacemaker system to make measurements of the electrical activity of myocardial tissue that are less susceptible to the effects of polarization potential. However, as with the foregoing inventions, this patent restricts the application of this technique to a period typically three milliseconds following the pacing signal, which itself is typically about one millisecond in length. The entire disclosure of this United States patent is hereby incorporated by reference into this specification.
Thus, these U.S. Pat. Nos. 5,957,857, 6,144,881, 5,871,512, and 5,330,512 disclose improvements in sensing heart activity in an individual with an implanted pacemaker, but they operate in a frequency range that is consistent with the pacing signal (typically about 1 kilohertz) and with the heartbeat (typically about 1 hertz).
U.S. Pat. No. 5,782,880 discloses a low energy pacing waveform for an implantable pacemaker, and suggests the use of a waveform different from the square-wave used in many pacemaker devices. This patent further discloses a pacing signal that is shaped so as to provide adequate safety factor in reliably pacing the cardiac tissue but that reduces the energy required to do so. The entire disclosure of this patent is hereby incorporated into this specification.
Reference may also be had to texts dealing with the topic of cardiac pacing. Thus, e.g., a text entitled xe2x80x9cCardiac Pacing for the Clinician,xe2x80x9d edited by M. Kusumoto and N. Goldschlager, and published by Lippincott Williams and Wilkins, 2001, contains several chapter sections that deal with the physiology of cardiac pacing and sensing, and that describe the methods used by contemporary manufacturers in dealing with the issues described above. In chapter 1 on page 9 of this text, the typical safety factors employed by physicians are described; these are a 2:1 safety factor for pacing signal voltage and a 3:1 safety factor for pacing signal duration. The text further teaches that the energy dissipated in a pacing signal is directly proportional to the duration of the signal and to the square of the voltage of this signal. Thus, typical practice results in a pacing signal that is 12-fold higher than a signal that would be adequate to initiate the heartbeat. This 12-fold excess is intended to provide a very reliable pacing system, but it also results in unnecessary and chronic damage to cardiac tissue, and in the wastage of more than 90% of the energy available in the pacemaker battery.
The above text further describes, on pages 12 and 13, the nature of the evoked potential that results from the heartbeat itself, and describes both its typical magnitude (10 to 20 millivolts) and its typical slew rate (1 to 4 volts per second).
The text further describes, on pages 18 to 24, the typical electrode structure used in pacing the heart and in sensing heart activity electrically. There is specific discussion of the ongoing debate relating to unipolar versus bipolar pacing, employing the use of one or two electrodes external to the pacemaker case, respectively.
Furthermore, and again referring to the text xe2x80x9cCardiac Pacing for the Clinician,xe2x80x9d and specifically to FIG. 1.15 on page 22, it is disclosed that the stimulation threshold of the heart increases substantially after initial implantation and use. It is generally acknowledged that damage to sensitive cardiac tissues is one of the primary causes of this increase, which in requires higher pacing voltages and safety factors.
It is an object of this invention to provide a cardiac assist device which is substantially more energy efficient than prior art cardiac assist devices and which, additionally, modifies its output in response to changing conditions in a heart.
In accordance with this invention, there is provided a cardiac assist system containing a device for delivering a first pacing pulse from said system to a heart, for ceasing the delivery of said first pacing pulse to said heart, for sensing whether the heart is beating, and for delivering a second pacing pulse from said system to the heart. Each of the first pacing pulse and the second pacing pulse are delivered to the heart within a period of less than about 100 milliseconds, and each said pacing pulse has a voltage of from about 2 to about 5 volts direct current. The time between the delivery of the first pacing pulse and the second pacing pulse is at least about 1 microsecond. The first pacing pulse and the second pacing pulse are delivered to the heart at a frequency of from about 2 kilohertz to about 5 megahertz; and they each have a duration of from about 0.1 microseconds to about 500 milliseconds. The cardiac assist system also contains a device for varying the voltage and/or the frequency of each of the pacing pulses.